The prior art is exemplified by U.S. Pat. Nos. 4,074,399; 4,081,883; and 4,422,220. The present invention is an improvement on my prior U.S. Pat. No. 4,074,399 and distinguishes over the above patents which do not have sliders with the same low-profile and with the same locking spring that is pre-loaded on a slider body in the assembled position.